This invention relates to apparatus for maintaining electrical continuity through an insulated electrical conductor employed in a drill string. The invention can be used in wellbore telemetry operations or other operations where electric energy is transmitted between subsurface locations and the surface. It particularly relates to an improved method and apparatus for storing insulated electrical conductor downhole as a coil in a storage container.
In the drilling of oil wells, gas wells or other boreholes, it frequently is desirable to transmit electric energy between subsurface and surface locations. One particularly important method receiving considerable attention in recent years is found in the use of a wellbore telemetry system designed to sense, transmit and receive information indicative of a subsurface condition or subsurface position. This system may include a downhole sensor mounted in the drill string above the drill bit. Processes using this generic apparatus have become known in the art as "logging while drilling" or "measurement while drilling", or simply "MWD".
A major problem associated with wellbore telemetry systems has been that of providing reliable means for transmitting an electric signal between the subsurface and surface locations. This problem can best be appreciated by considering the manner in which rotary drilling operations are normally performed. In conventional rotary drilling, a borehole is advanced by rotating a drilling string provided with a drill bit at its lower end. Sections of drill pipe, usually about 30 feet long, are added to the drill string, one at a time, as the borehole is advanced in increments. In adapting an electric telemetry system to rotary drilling equipment, the means for transmitting the electric signal through the drill string must allow the connection of additional pipe sections to the drill string as the borehole is advanced.
An early approach to the problem involved the use of continuous electric cable which was adapted to be lowered inside the drill string and to make contact with a subsurface terminal. This technique, however, requires withdrawing the cable from the drill string each time a pipe section was added to the drill string. Another approach involves the use of special drill pipe. Each pipe section of the special pipe is provided with an electric conductor having connectors at its opposite ends. Electrical continuity is maintained across the junction of two pipe sections by permitting a connector of one section to contact a connector on the adjacent pipe section (See U.S. Pat. Nos. 3,518,608 and 3,518,609). Disadvantages of this system include the high cost of the special pipe sections, the need for a large number of electric connections (one at each joint), and the difficulty of maintaining insulation of the electric connectors at each joint.
Still another approach involves the use of a cable section mounted in each pipe section (See U.S. Pat. No. 2,748,358). The cable sections are connected together as pipe sections are added to the drill string. Each cable section is normally made slightly longer than its associated pipe section, with the result that a small amount of slack is present in the conductor string at all times. Drilling fluid flowing through the drill string exerts a fluid drag on the loose cable and tends to damage the connectors and snarl the cable.
Another development in cable systems for wellbore telemetry operations is described in U.S. Pat. No. 3,825,078 on "Method of Mounting and Maintaining an Electric Conductor in a Drill String" and its divisional U.S. Pat. No. 3,913,688 on "Apparatus for Mounting Electric Conductor in a Drill String." The cable system disclosed in these patents employs a looped cable which permits the cable string to be extended as the drill string is lengthened. Experience with this system has indicated that the overlapped cable sometimes becomes entangled as a result of pipe rotation or fluid flow in the pipe setting. Although this patent suggests that downhole storage of coiled electric conductor is possible, it does not provide any method by which it can be done.
A method for preventing looped cable from tangling is described in U.S. Pat. No. 3,825,079 on "Method For Mounting An Electric Conductor in a Drill String" and its divisional U.S. Pat. No. 3,918,537 on "Apparatus for Maintaining an Electric Conductor in a Drill String." In accordance with one embodiment of these patents, entanglement of the overlapped cable is minimized by providing a track between a fixed upper support and lower axially movable weight. The support and the weight maintain the cable in an overlapped configuration, allowing stored cable to be withdrawn as drilling progresses, but minimize twisting of the looped cable. However, this system is cumbersome since it requires long track sections which must be lowered into and withdrawn from the pipe string. Furthermore, the length of cable which can be stored is somewhat limited.
An apparatus for storing coiled electric cable within a storage container inside a drill string is disclosed in U.S. Pat. No. 3,904,840 entitled "Wellbore Telemetry Apparatus." The cable within this apparatus is incrementally dispensed on demand during drilling operations via the use of clutch and gripper means. When tension on the conductor reaches some predetermined value, the gripper releases thereby allowing an amount of coiled cable to leave the storage container. The apparatus has no provision for self-restoration of the coiled cable. The coil itself is not disclosed to have any "spring" to it. Consequently, the coil must be returned to its storage container in a special operation on the surface.
An additional improvement to the operability of the looped cable methods noted above is described in U.S. Pat. No. 3,957,118 on "Cable System for Use in a Pipe String and Method for Installing and Using the Same." A disengageable cable gripping device is positioned on the pulley in the fixed upper support thereby maintaining tension on the portion of the conducting cable between the subsurface location and that upper support pulley. By maintaining this lower cable in tension, the tendency of the overlapping portions of the cable to tangle is substantially reduced.
Another disclosed improvement in the cable-gripping method entails the use of a device which prevents the upper and lower pulley assemblies from rotating with respect to each other. U.S. Pat. No. 4,098,342 additionally describes the desirability of placing cable guides at various intervals between the upper and lower pulley assemblies.
The disclosure in U.S. Pat. No. 4,181,184, to Scherbatskoy, relates to a method of storing unordered telemetry cable in the drill string. This method calls for using a softwire conductor having an insulation coating which is sufficiently flexible to permit random storage of the conductor within the drill pipe and yet springy enough to make possible a reasonably smooth extension of the wire after storage.
The downhole storage of very short length of cable is suggested in U.S. Pat. No. 2,706,616. The patent discloses a jar made up of two rigid telescoping tubes forming a cavity having therein a coiled conductor cable. The invention permits operation of the jar in a conductor string without effecting the electrical circuit through the equipment. The coiled conductor is not self-restoring after release nor is it mounted in a flexible housing.
Clearly, each of these inventions requires significant care in achieving successful downhole storage of electric cable. Only a few of these inventions provide for self-restoring cable storage during drilling. Most rely on the effect of gravity to prevent entanglement of the overlapping segments of the cable.